Dispersive Migration

Ring recoveries resulting from post-fledging dispersal usually drop off with increasing distance from the origin, as in natal dispersal, but they may extend further in some years, regions or habitats than in others, and further in one sex (usually females) than the other. In many resident species, recoveries come from greater average distances in successive months from fledging into winter, and then towards spring they come from localities progressively closer to the origin. The out-and-back movement patterns that are implied are apparent in a wide range of 'resident' species in Britain, including Great Tit Parus major, Blue Tit P. caeruleus, Song Thrush Turdus philomelos, Greenfinch Carduelis chloris, Chaffinch Fringilla coelebs, Common Murre Uria aalge, Herring Gull Larus argentatus, Great Black-backed Gull Larus marinus, European Shag Phalacrocorax aristotelis, Great Cormorant P. carbo, Grey Heron Ardea cinerea, Common Buzzard Buteo buteo and Mute Swan Cygnus olor (Wernham et al. 2002).

Such patterns could result from birds moving first progressively outward from their natal sites and then, as the next breeding season approaches, moving back towards their natal sites. Or they could result from birds that disperse furthest being more likely to die (not implausible if they are subordinate to others), and becoming less represented in the samples from late winter and spring. However, an outward-return movement has been confirmed independently in many species from repeated captures or sightings of live individuals, and also from radio-tracking studies (e.g. Spruce Grouse Dendragapus canadensis, Herzog & Keppie 1980, Schroeder 1985; Blue Grouse D. obscurus, Cade & Hoffman 1993; Greater Prairie Chicken Tympanuchus cupido, Schroeder & Braun 1993; Red Kite Milvus milvus, Evans et al. 1999; Great Bustard Otis tarda, Morales et al. 2000; Common Buzzard Buteo buteo, R. Kenward & S. Walls, in Wernham et al. 2002). In all these species, individuals moved outward from their natal or nesting areas after the breeding season, settled elsewhere over winter, and then moved back in time for the next breeding season. In other parts of their range, some of these species perform directed migrations.

Because this dispersal is a back-and-forth seasonal movement, consistent in timing from year to year, it parallels migration. But at the population level it differs in that: (1) it is not directional (unless direction is imposed by landscape or coastline); (2) the movements are also relatively short - say mostly in the range of a few kilometres or tens of kilometres (at least in passerines and game birds); (3) recoveries drop off rapidly with increasing distance rather than concentrating in a specific distant wintering area; and (4) all or most individuals remain year-round within the breeding range of the species (although some seabirds may disperse far from their colonies to remote feeding areas). The term dispersive migration seems appropriate for this type of movement, emphasising that it is seasonal, in various directions, and involves outward and return stages. While not obviously associated with seasonal changes in food supplies, one function of the movement is presumably to seek out better feeding sites for the winter, or to exploit places that are suitable for wintering but not for breeding. It is obviously not related to latitudinal trends in food supplies.

In Chapter 1, I alluded to the fact that the different types of bird movements intergrade, and that no clear distinction separates dispersal from migration. Nowhere is this more apparent than in dispersive migration. In some populations of dispersive migrants, some individuals shuttle back and forth between regular breeding and wintering areas, and show fidelity to both sites (as in Spruce Grouse Dendragapus canadensis, Herzog & Keppie 1980). As in many other birds, males migrate in smaller proportion than females, leave somewhat later and return earli er to their breeding areas next spring. In these respects, the return movements resemble the partial migrations of other birds. There are also sex differences in the distances moved, and in some lekking species in which males play no part in parental care, males travel further, on average, than females, as in Blue Grouse (Cade & Hoffman 1993) and Great Bustard (Morales et al. 2000). Significant differences in the distances moved are also apparent between age classes in some species. For example, adult and yearling female Black Grouse Tetrao tetrix that were radio-tagged in their wintering sites moved median distances of 2.6 km and 9.2 km to breeding sites, and maximum distances of 29.6 and 33.2 km, respectively. Females from the same lek moved in different directions from one another, with no overall directional preference (Marjakangas & Kiviniemi 2005).

In Common Buzzards Buteo buteo and other large raptors, ring recoveries suggest that birds disperse in their first year, and then move back towards their natal area as they approach breeding age (R. Kenward & S. Walls, in Wernham et al. 2002, Haller 1982). However, the radio-tracking of 124 young Common Buzzards showed that the situation was more complicated than this (Walls & Kenward 1998). In one study, the young started by making long excursive flights when they were about 70 days old, after their feathers had hardened, and then explored up to 25 km from their nests without dispersing (i.e. continually returning to their natal nests). However, about half the young also dispersed right away from natal areas by their first October. These birds moved longer distances than the remaining birds which left later in winter and the following spring. After leaving their natal areas, birds often changed home range more than once before settling, but none moved after their third summer. In the spring, they made temporary visits back to their natal areas, earlier in each successive year until they bred, either there or, more frequently, where they had settled in winter (Walls & Kenward 1998). Patterns of outward-return movement each year, of females moving further than males, and spending longer each year in the natal (future breeding) area with increasing age have also been recorded among Red Kites Milvus milvus (Evans et al. 1999), and some aspects in Spanish Imperial Eagles Aquila adalberti (Ferrer 1993). Again, these patterns of behaviour recorded from dispersive migrants during their first few years of life resemble those recorded from regular migrants wintering at lower latitudes (Chapter 15).

Dispersive migration on a much grander scale occurs among some seabirds which move in various directions away from their nesting colonies after breeding, but often remain within the same sea-water zones as they use in the nesting season (Figure 17.6). This has been indicated by ring recoveries from many species, and more convincingly by use of geolocation loggers attached to birds at their breeding colonies. This latter method has been used on adult Wandering Albatrosses Diomedia exulans nesting on the Crozet Islands in the southern Indian Ocean (Weimerskirch & Wilson 2000). After the young have fledged, the adults of this species leave the foraging areas frequented while breeding and head for sea areas 1500-8500 km away, elsewhere in the southern Indian Ocean or in the southwest Pacific. Here they spend the next year, returning to their nesting colonies for the following year, as they breed only every second year, being replaced in the nesting colonies during their away-years by other individuals. The ocean areas they occupy range from subtropical-tropical waters (females) to sub-Antarctic-Antarctic waters (males), and individuals probably visit the same areas in each sabbatical (Weimerskirch & Wilson 2000).

Similarly, 22 adult Grey-headed Albatrosses Thalassarche chrysostoma, studied by use of geolocation loggers, were followed throughout the approximately 18-month interval between successive breeding attempts on South Georgia (Croxall et al. 2005). During their sabbaticals, these birds showed three distinct dispersal strategies: (1) some stayed in the southwest Atlantic in an enlarged version of the breeding home range; (2) others made return movements (mainly eastward and back from the colony) to a specific region of the southwest Indian Ocean; and (3) yet others made one or two eastward heading journeys around the earth, foraging in various regions en route (the fastest in just 46 days) (Figure 17.7). Females more often remained in a restricted range, while most males performed at least one round-the-earth trip. The timing of journeys was generally well synchronised between individuals, and of consistent duration, and most made the same journeys in different years. Again, these birds on their dispersive migrations remained throughout within the same latitudinal band, and their round-the-earth journeys followed the prevailing winds.

Figure 17.6 Dispersive migration of two species of seabirds, based mainly on recoveries of birds ringed as nestlings in Britain (Wernham et al. 2002). (a) Common Guillemot Uria aalge. (b) Black-legged Kittiwake Rissa tridactyla. Modified from Flegg (2004).

Figure 17.7 Representative migration routes of four Grey-headed Albatrosses (A-D) Thalassarche chrystostoma in the 18 months between successive breeding attempts. White - winter; dark - summer; dotted lines link locations obtained before and after the equinox. From Croxall et al. (2005). Reprinted with permission from AAAS.

Figure 17.7 Representative migration routes of four Grey-headed Albatrosses (A-D) Thalassarche chrystostoma in the 18 months between successive breeding attempts. White - winter; dark - summer; dotted lines link locations obtained before and after the equinox. From Croxall et al. (2005). Reprinted with permission from AAAS.

Similar features involving broad-scale site-fidelity and consistency in timing of movements were also noted in Black-browed Albatrosses Thalassarche melano-phrys from South Georgia, which spent their sabbaticals off southwest Africa or Australia (Phillips et al. 2005). The migrations of all three albatross species could be said to be food-related, as birds moved from near breeding areas where competition was intense to other areas with abundant food away from colonies, but they achieved this by predominantly longitudinal rather than latitudinal shift. Moreover, individuals from the same colony did not all depart in the same direction as one another, and some of these directions differed from those taken by individuals from other colonies. For these reasons, it is hard to imagine that the directions taken by individual albatrosses have an innate basis, and more likely that cultural transmission and personal experience leads birds to focus on particular feeding areas (rather like moult migration in ducks, Chapter 16). Their movements all fall within the category of dispersive migration, as defined here.

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